Jitter

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In electronics and telecommunications, jitter is the deviation from true periodicity of a presumably periodic signal, often in relation to a reference clock signal. In clock recovery applications it is called timing jitter. [1] Jitter is a significant, and usually undesired, factor in the design of almost all communications links.

Contents

Jitter can be quantified in the same terms as all time-varying signals, e.g., root mean square (RMS), or peak-to-peak displacement. Also, like other time-varying signals, jitter can be expressed in terms of spectral density.

Jitter period is the interval between two times of maximum effect (or minimum effect) of a signal characteristic that varies regularly with time. Jitter frequency, the more commonly quoted figure, is its inverse. ITU-T G.810 classifies jitter frequencies below 10 Hz as wander and frequencies at or above 10 Hz as jitter. [2]

Jitter may be caused by electromagnetic interference and crosstalk with carriers of other signals. Jitter can cause a display monitor to flicker, affect the performance of processors in personal computers, introduce clicks or other undesired effects in audio signals, and cause loss of transmitted data between network devices. The amount of tolerable jitter depends on the affected application.

Metrics

For clock jitter, there are three commonly used metrics:

Absolute jitter
The absolute difference in the position of a clock's edge from where it would ideally be.
Period jitter (a.k.a. cycle jitter)
The difference between any one clock period and the ideal or average clock period. Period jitter tends to be important in synchronous circuitry such as digital state machines where the error-free operation of the circuitry is limited by the shortest possible clock period (average period less maximum cycle jitter), and the performance of the circuitry is set by the average clock period. Hence, synchronous circuitry benefits from minimizing period jitter, so that the shortest clock period approaches the average clock period.
Cycle-to-cycle jitter
The difference in duration of any two adjacent clock periods. It can be important for some types of clock generation circuitry used in microprocessors and RAM interfaces.

In telecommunications, the unit used for the above types of jitter is usually the unit interval (UI) which quantifies the jitter in terms of a fraction of the transmission unit period. This unit is useful because it scales with clock frequency and thus allows relatively slow interconnects such as T1 to be compared to higher-speed internet backbone links such as OC-192. Absolute units such as picoseconds are more common in microprocessor applications. Units of degrees and radians are also used.

In the normal distribution one standard deviation from the mean (dark blue) accounts for about 68% of the set, while two standard deviations from the mean (medium and dark blue) account for about 95% and three standard deviations (light, medium, and dark blue) account for about 99.7%. Standard deviation diagram.svg
In the normal distribution one standard deviation from the mean (dark blue) accounts for about 68% of the set, while two standard deviations from the mean (medium and dark blue) account for about 95% and three standard deviations (light, medium, and dark blue) account for about 99.7%.

If jitter has a Gaussian distribution, it is usually quantified using the standard deviation of this distribution. This translates to an RMS measurement for a zero-mean distribution. Often, jitter distribution is significantly non-Gaussian. This can occur if the jitter is caused by external sources such as power supply noise. In these cases, peak-to-peak measurements may be more useful. Many efforts have been made to meaningfully quantify distributions that are neither Gaussian nor have a meaningful peak level. All have shortcomings but most tend to be good enough for the purposes of engineering work. Note that typically, the reference point for jitter is defined such that the mean jitter is 0.[ citation needed ]

In computer networking, jitter can refer to packet delay variation, the variation (statistical dispersion) in the delay of the packets.

Types

One of the main differences between random and deterministic jitter is that deterministic jitter is bounded and random jitter is unbounded. [3] [4]

Random jitter

Random Jitter, also called Gaussian jitter, is unpredictable electronic timing noise. Random jitter typically follows a normal distribution [5] [6] due to being caused by thermal noise in an electrical circuit or due to the central limit theorem. The central limit theorem states that composite effect of many uncorrelated noise sources, regardless of the distributions, approaches a normal distribution. [7]

Deterministic jitter

Deterministic jitter is a type of clock or data signal jitter that is predictable and reproducible. The peak-to-peak value of this jitter is bounded, and the bounds can easily be observed and predicted. Deterministic jitter has a known non-normal distribution. Deterministic jitter can either be correlated to the data stream (data-dependent jitter) or uncorrelated to the data stream (bounded uncorrelated jitter). Examples of data-dependent jitter are duty-cycle dependent jitter (also known as duty-cycle distortion) and intersymbol interference.

Total jitter

nBER
6.410−10
6.710−11
710−12
7.310−13
7.610−14

Total jitter (T) is the combination of random jitter (R) and deterministic jitter (D) and is computed in the context to a required bit error rate (BER) for the system: [8]

T = Dpeak-to-peak + 2nRrms,

in which the value of n is based on the BER required of the link.

A common BER used in communication standards such as Ethernet is 10−12.

Examples

Sampling jitter

In analog-to-digital and digital-to-analog conversion of signals, the sampling is normally assumed to be periodic with a fixed period—the time between every two samples is the same. If there is jitter present on the clock signal to the analog-to-digital converter or a digital-to-analog converter, the time between samples varies and instantaneous signal error arises. The error is proportional to the slew rate of the desired signal and the absolute value of the clock error. The effect of jitter on the signal depends on the nature of the jitter. Random jitter tends to add broadband noise while periodic jitter tends to add errant spectral components, "birdys". In some conditions, less than a nanosecond of jitter can reduce the effective bit resolution of a converter with a Nyquist frequency of 22 kHz to 14 bits. [9]

Sampling jitter is an important consideration in high-frequency signal conversion, or where the clock signal is especially prone to interference.

In digital antenna arrays ADC and DAC jitters are the important factors determining the direction of arrival estimation accuracy [10] and the depth of jammers suppression. [11]

Packet jitter in computer networks

In the context of computer networks, packet jitter or packet delay variation (PDV) is the variation in latency as measured in the variability over time of the end-to-end delay across a network. A network with constant delay has no packet jitter. [12] Packet jitter is expressed as an average of the deviation from the network mean delay. [13] PDV is an important quality of service factor in assessment of network performance.

Transmitting a burst of traffic at a high rate followed by an interval or period of lower or zero rate transmission may also be seen as a form of jitter, as it represents a deviation from the average transmission rate. However, unlike the jitter caused by variation in latency, transmitting in bursts may be seen as a desirable feature,[ citation needed ] e.g. in variable bitrate transmissions.

Video and image jitter

Video or image jitter occurs when the horizontal lines of video image frames are randomly displaced due to the corruption of synchronization signals or electromagnetic interference during video transmission. Model-based dejittering study has been carried out under the framework of digital image and video restoration. [14]

Testing

Jitter in serial bus architectures is measured by means of eye patterns. There are standards for jitter measurement in serial bus architectures. The standards cover jitter tolerance, jitter transfer function and jitter generation, with the required values for these attributes varying among different applications. Where applicable, compliant systems are required to conform to these standards.

Testing for jitter and its measurement is of growing importance to electronics engineers because of increased clock frequencies in digital electronic circuitry to achieve higher device performance. Higher clock frequencies have commensurately smaller eye openings, and thus impose tighter tolerances on jitter. For example, modern computer motherboards have serial bus architectures with eye openings of 160 picoseconds or less. This is extremely small compared to parallel bus architectures with equivalent performance, which may have eye openings on the order of 1000 picoseconds.

Jitter is measured and evaluated in various ways depending on the type of circuit under test. [15] In all cases, the goal of jitter measurement is to verify that the jitter will not disrupt normal operation of the circuit.

Testing of device performance for jitter tolerance may involve injection of jitter into electronic components with specialized test equipment.

A less direct approach—in which analog waveforms are digitized and the resulting data stream analyzed—is employed when measuring pixel jitter in frame grabbers. [16]

Mitigation

Anti-jitter circuits

Anti-jitter circuits (AJCs) are a class of electronic circuits designed to reduce the level of jitter in a clock signal. AJCs operate by re-timing the output pulses so they align more closely to an idealized clock. They are widely used in clock and data recovery circuits in digital communications, as well as for data sampling systems such as the analog-to-digital converter and digital-to-analog converter. Examples of anti-jitter circuits include phase-locked loop and delay-locked loop.

Jitter buffers

Jitter buffers or de-jitter buffers are buffers used to counter jitter introduced by queuing in packet-switched networks to ensure continuous playout of an audio or video media stream transmitted over the network. [17] The maximum jitter that can be countered by a de-jitter buffer is equal to the buffering delay introduced before starting the play-out of the media stream. In the context of packet-switched networks, the term packet delay variation is often preferred over jitter.

Some systems use sophisticated delay-optimal de-jitter buffers that are capable of adapting the buffering delay to changing network characteristics. The adaptation logic is based on the jitter estimates computed from the arrival characteristics of the media packets. Adjustments associated with adaptive de-jittering involves introducing discontinuities in the media play-out which may be noticeable to the listener or viewer. Adaptive de-jittering is usually carried out for audio play-outs that include voice activity detection that allows the lengths of the silence periods to be adjusted, thus minimizing the perceptual impact of the adaptation.

Dejitterizer

A dejitterizer is a device that reduces jitter in a digital signal. [18] A dejitterizer usually consists of an elastic buffer in which the signal is temporarily stored and then retransmitted at a rate based on the average rate of the incoming signal. A dejitterizer may not be effective in removing low-frequency jitter (wander).

Filtering and decomposition

A filter can be designed to minimize the effect of sampling jitter. [19]

Jitter signal can be decomposed into Intrinsic Mode Functions (IMFs), which can be further applied for filtering or dejittering.[ citation needed ]

See also

Related Research Articles

Signal processing Academic subfield of electrical engineering

Signal processing is an electrical engineering subfield that focuses on analysing, modifying, and synthesizing signals such as sound, images, and scientific measurements. Signal processing techniques can be used to improve transmission, storage efficiency and subjective quality and to also emphasize or detect components of interest in a measured signal.

Analog-to-digital converter System that converts an analog signal into a digital signal

In electronics, an analog-to-digital converter is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide an isolated measurement such as an electronic device that converts an input analog voltage or current to a digital number representing the magnitude of the voltage or current. Typically the digital output is a two's complement binary number that is proportional to the input, but there are other possibilities.

Eye pattern

In telecommunication, an eye pattern, also known as an eye diagram, is an oscilloscope display in which a digital signal from a receiver is repetitively sampled and applied to the vertical input, while the data rate is used to trigger the horizontal sweep. It is so called because, for several types of coding, the pattern looks like a series of eyes between a pair of rails. It is a tool for the evaluation of the combined effects of channel noise, dispersion and intersymbol interference on the performance of a baseband pulse-transmission system.

Digital-to-analog converter Device that converts a digital signal into an analog signal

In electronics, a digital-to-analog converter is a system that converts a digital signal into an analog signal. An analog-to-digital converter (ADC) performs the reverse function.

Communication channel

A communication channel refers either to a physical transmission medium such as a wire, or to a logical connection over a multiplexed medium such as a radio channel in telecommunications and computer networking. A channel is used to convey an information signal, for example a digital bit stream, from one or several senders to one or several receivers. A channel has a certain capacity for transmitting information, often measured by its bandwidth in Hz or its data rate in bits per second.

Sound can be recorded and stored and played using either digital or analog techniques. Both techniques introduce errors and distortions in the sound, and these methods can be systematically compared. Musicians and listeners have argued over the superiority of digital versus analog sound recordings. Arguments for analog systems include the absence of fundamental error mechanisms which are present in digital audio systems, including aliasing and quantization noise. Advocates of digital point to the high levels of performance possible with digital audio, including excellent linearity in the audible band and low levels of noise and distortion.

In electronics and especially synchronous digital circuits, a clock signal oscillates between a high and a low state and is used like a metronome to coordinate actions of digital circuits.

This is an index of articles relating to electronics and electricity or natural electricity and things that run on electricity and things that use or conduct electricity.

Audio system measurements

Audio system measurements are a means of quantifying system performance. These measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers make them to ensure equipment is still working to specification, or to ensure that the cumulative defects of an audio path are within limits considered acceptable. Audio system measurements often accommodate psychoacoustic principles to measure the system in a way that relates to human hearing.

Sample and hold

In electronics, a sample and hold circuit is an analog device that samples the voltage of a continuously varying analog signal and holds its value at a constant level for a specified minimum period of time. Sample and hold circuits and related peak detectors are the elementary analog memory devices. They are typically used in analog-to-digital converters to eliminate variations in input signal that can corrupt the conversion process. They are also used in electronic music, for instance to impart a random quality to successively-played notes.

In electronic instrumentation and signal processing, a time to digital converter (TDC) is a device for recognizing events and providing a digital representation of the time they occurred. For example, a TDC might output the time of arrival for each incoming pulse. Some applications wish to measure the time interval between two events rather than some notion of an absolute time.

Pulse generator

A pulse generator is either an electronic circuit or a piece of electronic test equipment used to generate rectangular pulses. Pulse generators are used primarily for working with digital circuits, related function generators are used primarily for analog circuits.

In computer networking and telecommunications, TDM over IP (TDMoIP) is the emulation of time-division multiplexing (TDM) over a packet-switched network (PSN). TDM refers to a T1, E1, T3 or E3 signal, while the PSN is based either on IP or MPLS or on raw Ethernet. A related technology is circuit emulation, which enables transport of TDM traffic over cell-based (ATM) networks.

Delta-sigma modulation is a method for encoding analog signals into digital signals as found in an analog-to-digital converter (ADC). It is also used to convert high bit-count, low-frequency digital signals into lower bit-count, higher-frequency digital signals as part of the process to convert digital signals into analog as part of a digital-to-analog converter (DAC).

Clock skew is a phenomenon in synchronous digital circuit systems in which the same sourced clock signal arrives at different components at different times. The instantaneous difference between the readings of any two clocks is called their skew.

T.38 is an ITU recommendation for allowing transmission of fax over IP networks (FoIP) in real time.

Electronic circuit

An electronic circuit is composed of individual electronic components, such as resistors, transistors, capacitors, inductors and diodes, connected by conductive wires or traces through which electric current can flow. To be referred to as electronic, rather than electrical, generally at least one active component must be present. The combination of components and wires allows various simple and complex operations to be performed: signals can be amplified, computations can be performed, and data can be moved from one place to another.

Switching noise jitter Type of interference created by switching-mode power supplies

Switching Noise Jitter (SNJ) is the aggregation of variability of noise events in the time-domain on the supply bias of an electronic system, in particular with a voltage regulated supply bias incorporated with closed-loop (feedback) control, for instance, SMPS. SNJ is measurable using real-time spectral histogram analysis and expressed as a rate of occurrence in percentage. The existence of SNJ was firstly demonstrated and termed by TransSiP Inc in 2016 and 2017 at the Applied Power Electronics Conference (APEC), and reviewed with experts at Tektronix prior to be featured as a case study published by Tektronix. The discovery of SNJ was also featured in multiple articles published by Planet Analog magazine and EDN Network. Difficult to filter using conventional LC networks due to variability in both time and frequency domains, SNJ can introduce random errors in analog to digital conversion, affecting both data integrity and system performance in digital communications and location-based services

Deterministic Networking (DetNet) is an effort by the IETF DetNet Working Group to study implementation of deterministic data paths for real-time applications with extremely low data loss rates, packet delay variation (jitter), and bounded latency, such as audio and video streaming, industrial automation, and vehicle control.

Digital antenna array

Digital antenna array(DAA) is a smart antenna with multi channels digital beamforming, usually by using fast Fourier transform (FFT). The development and practical realization of digital antenna arrays theory started in 1962 under the guidance of Vladimir Varyukhin (USSR).

References

  1. Wolaver, Dan H. (1991). Phase-Locked Loop Circuit Design . Prentice Hall. p.  211. ISBN   978-0-13-662743-2.
  2. "FTB-8080 Sync Analyzer: Resolving Synchronization Problems in Telecom Networks" (PDF). EXFO. Application note 119. Archived from the original (PDF) on 2012-02-07. Retrieved 2012-08-05.
  3. Hagedorn, Julian; Alicke, Falk; Verma, Ankur (August 2017). "How to Measure Total Jitter" (PDF). Texas Instruments. SCAA120B. Retrieved 2018-07-17.
  4. "Understanding Jitter Calculations". Teledyne Technologies. July 9, 2014. Retrieved 2018-07-17.
  5. Hagedorn, Julian; Alicke, Falk; Verma, Ankur (August 2017). "How to Measure Total Jitter" (PDF). Texas Instruments. SCAA120B. Retrieved 2018-07-17.
  6. "Understanding Jitter Calculations". Teledyne Technologies. July 9, 2014. Retrieved 2018-07-17.
  7. Chow, Daniel. "Jitter Visualization, Part 1: Random Jitter". UBM Tech. Retrieved 12 April 2013.
  8. Stephens, Ransom. "The Meaning of Total Jitter" (PDF). Tektronix . Retrieved 2018-07-17.
  9. Puente León, Fernando (2015). Messtechnik. Springer. p. 332f. ISBN   978-3-662-44820-5.
  10. M. Bondarenko and V.I. Slyusar. "Influence of jitter in ADC on precision of direction-finding by digital antenna arrays. // Radioelectronics and Communications Systems. - Volume 54, Number 8, 2011.- Pp. 436 - 445.-" (PDF). doi:10.3103/S0735272711080061. S2CID   110506568.Cite journal requires |journal= (help)
  11. Bondarenko M.V., Slyusar V.I. "Limiting depth of jammer's suppression in a digital antenna array in conditions of ADC jitter.// 5th International Scientific Conference on Defensive Technologies, OTEH 2012. - 18 - 19 September, 2012. - Belgrade, Serbia. - Pp. 495 - 497" (PDF).
  12. Comer, Douglas E. (2008). Computer Networks and Internets. Prentice Hall. p. 476. ISBN   978-0-13-606127-4.
  13. Demichelis, C. (November 2002). IP Packet Delay Variation Metric for IP Performance Metrics (IPPM). IETF. doi: 10.17487/RFC3393 . RFC 3393.
  14. Kang, Sung-Ha; Shen, Jianhong (Jackie) (2006). "Video Dejittering by Bake and Shake". Image and Vision Computing. 24 (2): 143–152. doi:10.1016/j.imavis.2005.09.022.
  15. M. Bondarenko and V.I. Slyusar. "Methods for estimating the ADC jitter in noncoherent systems. // Radioelectronics and Communications Systems. - Volume 54, Number 10, 2011. – Pp. 536 - 545. - DOI: 10.3103/S0735272711100037" (PDF).
  16. Khvilivitzky, Alexander (2008). "Pixel Jitter in Frame Grabbers" . Retrieved 2015-03-09.
  17. Jitter Buffer
  18. PD-icon.svg This article incorporates  public domain material from the General Services Administration document: "dejitterizer".(Federal Standard 1037C)
  19. S. Ahmed; T. Chen. "Minimizing the effects of sampling jitters in wireless sensors networks".Cite journal requires |journal= (help)

Further reading